This invention relates to a process for producing a polymer of reduced molecular weight. Specifically, the invention relates to a process for producing a halogenated butyl rubber of reduced molecular weight.
The manufacture of butyl rubber by copolymerization of isobutylene with a small amount of isoprene at temperatures below -40.degree. C., usually below -80.degree. C., is well known in the art--see for example, U.S. Pat. No. 2,399,672. Halogenated butyl polymers have been known and commercially available for many years, and are prepared by halogenating butyl rubber. During the halogenation process, the isoprene units of the butyl rubber (I) react with the halogen (X2) to yield halogen-substituted units (II) and hydrogen halide as a by-product: ##STR1## Structure II, which is hereinafter referred to as the "EXO methylene configuration", is believed to be the predominant configuration of the halogen-substituted units in both commercial brominated and chlorinated butyl rubbers made by a solution process--see for example the Vanderbilt Rubber Handbook, by R. O. Babbit (R. T. Vanderbilt Company, Inc.); pages 102 and 133.
Certain polymers may be treated with ozone containing gas to produce polymers of lower molecular weight, such reactions being undertaken with the polymer in solution in a solvent--see for example, British Patent No. 984,071 and U.S. Pat. No. 3,392,154. It is also well known in the art that rubbery vulcanizates will crack when exposed to an atmosphere containing ozone but only when the rubbery vulcanizate is subjected to a strain such as by elongation. British Patent No. 883,791 teaches that a process for making graft-type polymers is to introduce oxygenated groups into the base polymer following which a polymerizable monomer is added and the oxygenated groups are decomposed to cause polymerization and grafting of the monomer onto the base polymer. One illustration of such a process is to masticate a polymer in the presence of ozone following which monomer is mixed with the ozonized polymer and the mixture is heated to cause grafting of the monomer onto the original polymer.
British Patent No. 884,448 teaches a two-step process for improving the physical properties of a large variety of polymers. Specifically, the process comprises (i) contacting apolymer with ozone at a temperature less than 130.degree. C. and (ii) subjecting the ozonized polymer to a heat treatment in an oxygen-free environment at a temperature of from about 20.degree. to about 250.degree. C. The products of this process are claimed to possess improvements in tensile strength, tear resistance, abrasion resistance and a variety of other physical properties.